显示/隐藏动态组合框

我有两个由mysql查询生成的组合框。

当用户在第一个组合框中选择值时，需要显示第二个组合框，但如果没有选择，则不应显示第二个组合框。代码:

<?php
echo "<form method= '"post'" name='"formcombo'" action=''>";
echo "<select name='cat' onchange='"reload(this.form)'"><option value=''>Choose main category</option>";
while($cat2 = mysql_fetch_array($query2)) {
if($cat2['category_id']==@$category)
{
echo "<option value='$cat2[category_id]'>$cat2[category_name]</option></br>";}
else
{
 echo "<option value='$cat2[category_id]'>$cat2[category_name]</option>";
}
}
 echo "</select></br>";
 echo "<select name='subcat'><option value=''>Choose subcategory</option>";
 while($cat = mysql_fetch_array($query1)) {
 echo "<option value='$cat[subcat_name]'">$cat[subcat_name]</option>";
   }
echo "</select>";
echo "<input type='"submit'" value ='"Submit'">";
echo "</form>";
 ?>

我使用javascript生成组合框值:

<script type="text/javascript">
function reload(form){
var val=form.cat.options[form.cat.options.selectedIndex].value;
self.location='main.php?cat=' + val ;
}
</script>